A common rule of thumb is that about 3,500 calories equal one pound of body fat (roughly 7,700 calories per kilogram). A daily deficit of 500 calories therefore adds up to about 3,500 per week, or around one pound. This calculator divides the total weight to lose by your weekly loss to estimate the time.
Example: losing 20 lb with a 500 calorie daily deficit is 20 x 3,500 = 70,000 calories, which takes 140 days, or 20 weeks.
Many guidelines suggest losing about 0.5 to 1 kg (1 to 2 lb) per week. Very large deficits are hard to sustain and can cost muscle. Find your maintenance calories with the TDEE calculator and subtract your deficit from that.
As you get lighter you burn fewer calories, so a fixed deficit shrinks over time. Treat the result as a best-case estimate and recheck your numbers every few weeks.
This is a general estimate, not medical advice. Talk to a doctor before starting a diet, especially if you have a medical condition.
See also: TDEE calculator · BMI calculator · macro calculator